214 FREDERICK TILNEY 



vein line, but in those regions corresponding to the perimeso- 

 nephroic plexus of the younger stages, the intermediate element 

 is obviously the selected channel. The relations of this element 

 to the aorta in the 6 mm. specimen indicate the future line of 

 the azygos vessel. In the 13 mm. embryo the intermediate ele- 

 ment has become a more prominent channel than the post-cardi- 

 nal. A right azygos Vein, corresponding in position to the left 

 vessel, is formed in like manner, except that it establishes its 

 ultimate drainage connections by a series of cross anastomoses 

 with the left vein. These anastomoses, at first, are diffuse and, 

 in the 20 mm. embryo, appear as irregular channels between 

 the two azygos lines. The most cephalic channel is well defined 

 and larger than the rest. In the 23 mm. embryo the formation 

 of distinct cross vessels has been carried much further so that 

 these inter-azygos connecting lines are now arranged in seg- 

 mental series, are twelve in number, and occur at the junction 

 of each dorsal segmental vein with the azygos vessel of its respec- 

 tive side (fig. 10, 62). Another set of anastomosing vessels serves 

 to connect the two azygos channels with the sub-cardinal and 

 cardinal collateral lines. These anastomotic vessels are placed 

 along the sides of the aorta and thus correspond in their relations 

 to the mesial branches of the perimesenephroic plexus observed 

 in the younger stages. Two sets of these vessels may be dis- 

 tinguished, a cephalic anastomosis which traverses the anlage of 

 the supra-renal body and undergoes a gradual reduction in pass- 

 ing from the 13 mm. to the 23 mm. stage. This plexus forms 

 a connection between the azygos vein and the corresponding 

 side of the post-cava in its inter-renal segment. The second 

 plexus is much more extensive. It establishes a communication 

 between the azygos veins and cardinal collateral portions of 

 the post-cava (fig. 14, 61). The significance of this connection 

 in its bearing upon the possibilities of post-caval formation will 

 subsequently be discussed. That it gradually diminishes in sig- 

 nificance as growth proceeds is witnessed by the fact of its actual 

 reduction in passing from the 13 mm. to the 23 mm. embryo. 

 The post-cardinal veins thus concern themselves with the azygos 

 system, allowing the selection of the caval drainage line to fall 



